Selective catalytic reduction (SCR) systems are used to treat engine exhaust to provide a reduction in NOx emissions. Many SCR systems use ammonia as a reductant for reducing NOx to N2. A typical SCR system includes a substrate having a catalyst such as vanadium, titanium, platinum or other precious metal. In a typical NOx reduction reaction, the catalyst provided on the substrate promotes the reaction of ammonia with NOx to form N2 and H2O. This reaction is favored by the presence of excess oxygen.
In certain SCR systems, a urea-water solution is provided as a reductant source for use in the reduction of NOx. However, to function as an effective reductant, the urea-water solution must be decomposed into a useable reductant form (i.e., ammonia). To decompose the urea-water solution, the water in the solution is first removed through vaporization. Next, the urea decomposes via thermolysis to form isocyanic acid (HNCO) and ammonia (NH3). Finally, the HNCO decomposes via hydrolysis to form NH3 and carbon dioxide.
A problem with urea-based SCR systems is that urea and urea-by-product (e.g., cyanuric acid, biuret, melamine, ammelide, and ammeline) deposits can be formed within the exhaust system when decomposition of the injected urea-water solution is incomplete. This deposition of urea and urea-by-products is particularly problematic when the urea-water solution sprayed into the exhaust system is exposed to relatively low temperatures.